Question

I am writing a data transfer application using a servlet and would like to be able to send an error response if a problem occurs after the servlet response has been written to. Is that possible?

My issue is that I will be sending large compressed csv files that are created from data read from a database. Everything is done with streams so it is possible that an error could occur in the creation of the csv file after the servlet response has been written to. I have seen it happen.

I’ve noticed that this is only a problem after the servlet OutputStream has been flushed. If it has not been flushed I can send an error response but not after. Since I am dealing with large amounts of data it is not feasible to send everything in one go.

Answer 1

I am writing a data transfer application using a servlet and would like to be able to send an error response if a problem occurs after the servlet response has been written to. Is that possible?

Not from the server side on. The server cannot take the already flushed bytes back from the client. This is a point of no return. I assume that this concerns a different exception than IOException on the response’s Writer or OutputStream.

If it were HTML (even though this is a poor practice; HTML belongs in JSP), you could print some JS code which forces a location change like so:

try {
    writer.write(someHtml);
} catch (SomeException e) {
    writer.write("<script>window.location = 'error.jsp';</script>");
    // ...
}

But this is not possible in non-HTML responses. You’d really need to buffer the entire response in memory or on (temp) disk beforehand. If buffering went flawlessly, then you can pipe it to the response again.

try {
    processAndSaveInMemoryOrTempDiskFile(someData, byteArrayOrFileLocation);
} catch (SomeException e) {
    throw new ServletException(e, "Processing some data failed.");
}

copyFromMemoryOrTempDiskToResponse(byteArrayOrFileLocation, writer);